In the PLIST, you can then have:
${PLIST.pam}lib/security/pam_winbind.so
${PLIST.nss)lib/nss_winbind.so

As I said some time ago, prefixing path part with a variable loses
grep'ability. I still think the syntax like distrib/sets/lists would
be better.
lib/security/pam_winbind.so pam
lib/nss_winbind.so nss

This does run into problems with files with spaces in them. To be
honest, I don't know if pkgsrc even supports files with spaces in them,
but I think that at least the above would be confusing to the code.

I suspect that when we solve the sub-packaging problem for PLISTs that
we will simply re-use the solution for this problem. For now, I just
wanted to keep mostly the same well-understood mechanism used in current
PLISTs.